The invention is based on a device for operating a windshield wiper.
A device for controlling a windshield wiper system is already known (from German Patent Disclosure DE 41 41 348 A1), which automatically adapts the wiping cycle frequency to qualitatively and/or quantitatively varying states of the precipitation on the window. To that end, the signals of an optical rain sensor are evaluated by a circuit arrangement. A wiping cycle is initiated when the signals undershoot a particular switching threshold. Each time the wiper sweeps over the rain sensor, the signal increases again, up to a maximum signal value, which is then stored in memory as a reference value for the next wiping cycle. The switching threshold is defined at 95%, as a function of this reference value.
A substantial disadvantage of this control arrangement is that after a defined period of time has elapsed, a new wiping cycle is tripped if during this time period no new precipitation has formed to a sufficient extent, and if the signal value of the sensor device does not undershoot the switching threshold. A new reference value is thus formed, because the old one in the meantime may no longer be usable, because of the temperature drift of the sensor signal. Yet this means that the wiper will be wiping a dry or nearly dry window.
Another resultant disadvantage is that signals generated by fog or drizzle cannot be distinguished from the signal change caused by an increase in temperature of the sensor: Both effects cause the signal value to undershoot the switching threshold.
Still another disadvantage is that the reference value is written in anew for each wiping operation and used to determine the switching threshold for the next wiping cycle. Heavy soiling of the window, for instance, leads to a lower reference value and thus a lower switching threshold, so that the wiping operation is not tripped until there is a heavier total coating of both soil and moisture on the window. This heavier coating impairs the driver's vision and is a safety risk.
It is therefore an object of the present invention to provide a device for operating a windshield wiper, which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a device for operating a windshield wiper, in which incremental values are allocated to the measured values by a control unit, and a mean value of at least two successive measured values is formed as the incremental value, and the respective difference between two successive incremental values is to be added with a sign + or - in the memory to a sum of differences formed previously in the same way, and finally when the threshold is reached by the sum stored in the memory, the control unit trips a wiper operating mode.